This invention relates to an electrostatic capacitor type sensor and a pulse phase adjusting circuit for use with the such a sensor, and more particularly, to electrostatic capacitors used for the detection of water level in pools or rivers, the detection of a substance at a tip portion of hands of a robot, the detection of substances on a belt conveyor in a factory or the like, and so on. The present invention also relates to a pulse transmission circuit arranged such that a series circuit of a capacitor and a resistor is connected to a charging-discharging circuit and to a pulse generator, and a junction between the above-mentioned capacitor and resistor is connected to gate of an FET, and particularly to a circuit for adjusting phase of a pulse signal in such a circuit, which can be widely applied to a water gauge, detection of a substance at the tip of robot hands or on a belt conveyor using an electrostatic capacitance type sensor.
Various types of sensors have hitherto been developed for the detection of water level, or the detection of substances at the the tip portion of robot hands. One of such sensors utilizes the variation in electrostatic capacitance. This sensor detects the existence or an approaching state of an object to be detected by using the variation in distributed capacitance, i.e. stray capacitance, at the time the object approaches a capacitance portion of the sensor, where the variation result in the change in the resonance frequency of a resonance circuit including said capacitor.
In such measurement, since it is necessary to select the resonance frequency at a high value, such as several kHz to several MHz, in order to increase the detection accuracy, the electrostatic capacitance of the sensor, which is a component determining the resonance frequency, has to be reduced to an extremely small value. Furthermore, the electrostatic capacitance has to be reduced for ensuring high Q. Normally, the electrostatic capacitance of such a sensor ranges from 0.l pF to 5 pF. Therefore, in a conventional electrostatic capacitance type sensor, a detection electrode 10 facing an object to be detected, a ground electrode 12 facing the detection electrode, and an additional electrode 14 facing the ground electrode are arranged having a given space therebetween as shown in FIG. 10. Especially, the ground electrode 12 and the additional electrode 14 have a structure that a hollow cylindrical member is arranged axially so that adjacent electrodes do no face each other at their surfaces. When hollow inside of the ground electrode 12 and the additional electrode 14 is filled with a synthetic resin or the like, the resonance frequency and Q drop due to the increase in electrostatic capacitance. For this reason, the above-mentioned hollow portions are left hollow when these electrodes are fixed in a casing of the sensor.
Although the change of electrostatic capacitance due to the variation in ambient temperature is small in the conventional sensors having such structure, when increasing the sensitivity of a measuring circuit to the extreme degree, the resonance frequency is affected by a slight change in the electrostatic capacitance. Therefore, it is difficult to sufficiently increase the detection sensitivity. Although such influence by the change in electrostatic capacitance due to the change in temperature can be bettered to some extent by adding a capacitor having an inverse temperature characteristic, such measures are still insufficient and therefore, there have been no effective compensation measures which prevent the detection sensitivity from dropping due to temperature change.
Furthermore, since the inside of the hollow cylindrical electrode provides a cavity, it has a drawback that strength against an impact is low.
In measuring apparatus, such as a water gauge, using an electrostatic capacitance type sensor, there is known apparatus in which resonance frequency is varied by the change in electrostatic capacitance so that the variation is detected. However, such conventional measuring apparatus has a drawback in connection with detection accuracy and temperature compensation. On the other hand, a measuring circuit of phase comparison type has recently been developed. This circuit detects the difference in electrostatic capacitances between two electrostatic capacitance devices as phase difference. More specifically, as shown in FIG. 15, a pulse signal is fed to a resistor R1 connected to a detection device C1, while the same pulse signal is fed to a resistor R2 connected to a comparison device C2, and phase variation caused from the change in electrostatic capacitance of C1 is detected by two inverter circuits 50 and 432 comprising CMOS IC of FET input type and a flip-flop 60 connected thereto.
In such method of detection of phase difference, it is necessary that the phase at C1 is matched with that at C2 as accurate as possible prior to the commencement of measurement or C1 and C2 are spaced apart as much as possible. Furthermore, it is desirable that the above-mentioned phase difference can be precisely controlled in advance depending on the difference in objects to be detected, measuring state or set threshold.
The phases of pulse signals passing through the above-mentioned C1 and c2 are respectively determined by the time constant (C1.times.R) of the series circuit of C1 and R1, and the time constant (C2.times.R2) of the series circuit of C2 and R2. Therefore, the above-mentioned phases are arbitrarily set through the adjustment of C1, R1, C2, R2. Generally speaking, continuous and wide range adjustment of the electrostatic capacitance of a capacitor is difficult, and therefore, it is only possible to stepwisely change the same or to obtain slight change by a trimmer capacitor. FIG. 16 shows one channel of the circuit of FIG. 15 schematically, and FIGS. 17 and 18 show structures that a variable resitor 210 or a trimmer capacitor 212 is added as phase adjusting means to the apparatus of FIG. 16. In the case of FIG. 15, phase difference can be freely set by manually adjusting the variable resistor 210 in the same manner as in FIG. 17.
In the above-mentioned conventional apparatus of FIG. 15, portions indicated by thick lines are required to be wired as short as possible. This is because stray capacitance or distributed capacitance may result in error or malfunction. This also applies to FIGS. 16 and 17. For this reason, the variable resistor 210 and the trimmer capacitor 212 of FIGS. 15, 17 and 18 could no be separately positioned to be remote from the sensor body including the capacitor C1. Therefore, when the position of the sensor is at an extremely low or high place, or in a complex apparatus, it is very difficult to perform phase adjustment, and in some cases such adjustment is substantially impossible. In this way, phase adjustment has hitherto been difficult in conventional apparatus.